The present invention relates to acoustic projectors, especially projectors for use in low frequency military and civilian sonar systems, and in particular to underwater acoustic projectors having highly stable performance with depth, improved frequency range and reduced manufacturing costs due to lower mechanical tolerances being required than in existing acoustic projectors.
Low frequency military and civilian sonar systems require compact, light weight, high power, efficient, wide bandwidth acoustic projectors whose performance is stable with depth and linear with drive voltage levels and which have a low manufacturing and maintenance cost.
Flextensional projectors are amongst the best ones presently available to meet the military and civilian sonar systems requirements, a known flextensional projector being the barrel stave type. The barrel stave projector (BSP) is a compact, low frequency underwater sound source which has applications in low frequency active (LFA) sonar and in underwater communications.
Variants of this known BSP have been built to optimise light weight, wide bandwidth, low frequency, high power, and improved electroacoustic efficiency. Efficiency is an especially critical parameter for the high power versions of the BSP because the driver is well insulated from the water thermally by a boot on the outer surface of projector that is required for waterproofing. The boot""s relatively poor thermal conductivity contributes to the difficulty in cooling the BSP. The BSPs are relatively costly to manufacture and maintain.
A one-piece flextensional shell projector is described by Christopher Purcell in U.S. Pat. No. 5,805,529. The surface of this projector is formed of a thin-walled one-piece inwardly concavely shaped shell containing corrugations running in the axial direction. This one-piece shell is slotless which eliminates the requirement for a boot. The shell is, however, relatively costly to manufacture since it is complex in shape and must be made to fine tolerances.
Canadian Patent 1,319,414 by Bryce Fanning et al that issued on Jun. 22, 1993 describes one type of a free-flooding piezoelectrically driven resonant-pipe projector (RPP) with vent holes in the pipe walls to broaden the response of certain cavity resonances and to increase the response between those resonances. The drive unit is a radially-poled lead zirconate-titanate cylinder with aluminium pipes extending into the center of the piezoelectric drive unit, the pipes being mechanically coupled to the drive unit. To accomplish the necessary acoustic coupling between the drive unit and pipes requires a close mechanical fit to couple the drive unit to the pipes. These resonant pipe projectors are partially free-flooding and can be operated at extreme depths because the drive unit is highly resistant to hydrostatic loading. However, the bandwidth is small and they are expensive to manufacture due to the close tolerances required.
It is an object of the invention to provide an acoustic projector with reduced depth sensitivity when submerged in water, improved frequency range and reduced manufacturing costs.
An acoustic projector, according to one embodiment of the present invention, comprises a pair of spaced apart end walls with an acoustic driver positioned between and coupled to the end walls, the driver having smaller cross-sectional dimensions than the end walls which have flared pipe waveguides extending outward from the driver, outer ends of the waveguides having a larger diameter than other portions of the waveguides.